Single strategy for scale expansion/limiting that works with transformations

[paleolimbot]

There are a number of issues that surround expansion and limiting, which is treated differently in various coordinate systems. It would be nice if specifying xlim and ylim were like specifying scale_(x|y)_*(limits = ...), and if scale expansion were identical for scale_(x|y)_continuous(trans = ...) and coord_trans((x|y) = ...).

Related issues:

• Scale expand value from scales ignored when coord_cartesian ylim is used #3270: interaction between xlim/ylim and expand
• NA limits in coords #2907: Allowing NA in xlim/ylim as one can do in the scale limits
• sqrt_trans, scale limit expansion, and missing breaks #980: breaks for expanded scales with transformations
• Secondary axis doesn't show axis ticks or labels with coord_trans #2990, Remove extra space created by coord_trans #3338: expansion behaviour in coord_trans()

I think these could almost all be handled in the relatively new ViewScale class, whose objects are currently being created here (this will probably change):

ggplot2/R/coord-cartesian-.r

Lines 139 to 161 in 6c2fe76

	view_scales_from_scale <- function(scale, coord_limits = NULL, expand = TRUE) {
	expansion <- if (expand) expand_default(scale) else expand_scale(0, 0)
	limits <- scale$get_limits()
	if (is.null(coord_limits)) {
	continuous_range <- scale$dimension(expansion, limits)
	} else {
	continuous_range <- range(scale$transform(coord_limits))
	continuous_range <- expand_range4(continuous_range, expansion)
	}
	aesthetic <- scale$aesthetics[1]
	view_scales <- list(
	view_scale_primary(scale, limits, continuous_range),
	sec = view_scale_secondary(scale, limits, continuous_range),
	arrange = scale$axis_order(),
	range = continuous_range
	)
	names(view_scales) <- c(aesthetic, paste0(aesthetic, ".", names(view_scales)[-1]))
	view_scales
	}

[paleolimbot]

In a slightly similar way, coordinate systems should probably be able to replace scale defaults that aren't explicitly set (should they?). Off the top of my head, coord_trans() should probably be able to coerce the (unset) breaks and labels on the transformed axes to trans$breaks or trans$labels, and there is one open issue on something similar with coord_fixed() (#2888)

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